This invention relates to a feedback control method and apparatus in a closed-loop transmit diversity system. More particularly, the invention relates to a feedback control method and apparatus in which a delay in transmit diversity that arises immediately after handover is avoided by calculating and feeding back antenna weight with respect to a handover-destination base station in advance of base-station changeover by handover.
In a closed-loop transmit diversity scheme, a radio base station of a cellular mobile communication system is provided with a plurality an antenna elements. A base station {circle around (1)} subjects the same transmit data signal to different amplitude and phase control based upon feedback information that is sent from a mobile station, and {circle around (2)} multiplexes pilot signals onto the transmit data that has undergone the amplitude and phase control and transmits the resultant signals using different antennas. {circle around (3)} The mobile-station side again decides the feedback information (amounts of amplitude and phase control) using downlink pilot signals, multiplexes the information onto an uplink channel signal and transmits the resultant signal to the side of the base station. The above-described operation is thenceforth repeated.
With closed-loop transmit diversity in W-CDMA, which is a third-generation mobile communication system, a scheme that uses two transmit antennas is employed. FIG. 13 is a diagram illustrating the system configuration in a case where two transmit antennas are used. Mutually orthogonal pilot patterns P1, P2 are generated in a pilot signal generator 11, the signals are incorporated into transmit data in combiners CB1, CB2 and transmitted from transmit antennas 10-1, 10-2, respectively. A channel estimation unit (not shown) on the receiving side of a mobile station correlates the receive pilots signals and corresponding known pilot patterns, whereby it is possible to estimate channel-impulse response vectors h1, h2 from the transmit antennas 10-1, 10-2 of the base station to a receive antenna 12 at the mobile station.
A control-amount calculation unit 13 uses these channel estimation values to calculate an amplitude and phase control vector (weight vector) w=[w1,w2]T of the transmit antennas 10-1, 10-2 of the base station, which vector maximizes power P indicated by Equation (1) below. The vector is quantized, multiplexed onto the uplink channel signal as feedback information and transmitted to the side of the base station. It should be noted that it is unnecessary to transmit both the values w1, w2, it being sufficient to transmit only the value w2 in a case where w1 is obtained as w1=1.P=wHHHw  (1)H=[h1,h2]  (2)
Here h1, h2, represent the channel-impulse response vectors from the transmit antennas 10-1 and 10-2, respectively. Further, the suffix H at the upper right of HH and wH indicates taking the Hermitian conjugate of H and w. The impulse response vector hi is expressed by the following equation, where L represents the length of the impulse response:hi=[hi1,hi2, . . . , hiL]T  (3)
When a soft handover is performed, the control vector w that maximizes the following equation instead of Equation (1) is calculated:P=wH(H1HH1+H2HH2+ . . . )w  (4)where Hk is the channel impulse response of the signal from a kth base station.
The mobile station calculates the weighting coefficients (weight vector) in the control-amount calculation unit 13, multiplexes the weighting coefficient onto the uplink transmit data as feedback information using a multiplexer 18 and transmits the information to the base station from a transmit antenna 14. At the base station the feedback information from the mobile station is received by a receive antenna 15, the weighting coefficients w1, w2, which are the control quantities, are extracted by a feedback information extraction unit 16, and an amplitude and phase controller 17 multiplies the downlink transmit data by the weighting coefficients w1, w2 using multipliers MP1, MP2 and controls the amplitude and phase of the signals transmitted from the transmit antennas 10-1, 10-2. As a result, the mobile station is capable of receiving the signals transmitted from the two diversity transmit antennas 10-1, 10-2 in an efficient manner.
Two methods are stipulated in W-CDMA, namely a mode 1, in which the weighting coefficient w2 is quantized to one bit, and a mode 2, in which the weighting coefficient w2 is quantized to four bits. In mode 1, 1-bit feedback information is transmitted every slot to perform control and therefore control speed is high. Accurate control cannot be achieved, however, because quantization is coarse. In mode 2, on the other hand, control is performed by 4-bit information and, hence, highly accurate can be achieved. However, feedback information is transmitted one bit at a time in each frame and one word of feedback information is transmitted by four slots. If the fading frequency is high, therefore, follow-up will not be possible and a degraded characteristic will result. Thus, in a case where the uplink channel signal transmission rate for transmitting the feedback information is limited, there is a trade-off relationship between control precision and fading-follow-up speed.
FIG. 14 is a diagram showing the structure of an uplink frame standardized by the 3rd Generation Partnership Project (referred to as “3GPP” below). A DPDCH data channel (Dedicated Physical Data Channel) on which only transmit data is transmitted and a DPCCH control channel (Dedicated Physical Control Channel) on which a pilot and control data such as feedback information are multiplexed and transmitted are multiplexed on real and imaginary axes by orthogonal codes. More specifically, one frame has a duration of 10 ms and is composed of 15 slots (slot #0 to slot #14). The DPDCH data channel is mapped to an orthogonal I channel of QPSK modulation and the DPCCH control channel is mapped to an orthogonal Q channel of QPSK modulation. Each slot of the DPDCH data channel (I channel) consists of n bits, and n varies in accordance with the symbol rate. Data of one or more transport channels can be multiplexed and transmitted, up to a maximum of six channels, on the DPDCH data channel. Each slot of the DPCCH control channel (Q channel) that transmits the control data consists of ten bits, has a symbol rate of a constant 15 ksps and transmits a pilot PILOT, transmission power control data TPC, a transport format combination indicator TFCI and feedback information FBI. The PILOT is utilized on the receiving side to perform channel estimation (estimation of propagation path characteristics) and when measuring SIR. The TFCI transmits the symbol speed of data, the number of bits per frame and the number of bits increased by repetition, etc. The FBI transmits the above-mentioned feedback information (weighting coefficients; weight vectors) for controlling the transmit diversity at the base station.
According to the specifications of Release 99 of W-CDMA, no consideration is given to a case where the number of transmit antennas is greater than two in order to avoid a decline in the transmission efficiency of the uplink channel owing to transmission of feedback information. However, if an increase in feedback information or a decline in update speed is allowed, expansion to three or more antennas is possible.
FIG. 15 is a diagram illustrating an example of an arrangement for a case where the number of transmit antennas is four. Structural elements in FIG. 15 similar to those shown in FIG. 13 are designated by like reference numerals and are not described again. In a case where there are N-number of transmit antennas (there are four transmit antennas 10-1 to 10-4 in the example of FIG. 15), N-number of mutually orthogonal pilot signals P1(t), P2(t), . . . PN(t) are transmitted by a radio base station using respective ones of different transmit antennas. The pilot signals are related as follows:∫Pi(t)Pj(t)dt=0 (i≠j)  (5)
Each pilot signal sustains its own amplitude and phase fluctuation ascribable to fading, and the combined signals are input to the receive antenna 12 of the mobile station. The channel estimation unit (not shown) in the receiver of the mobile station obtains the correlation between the receive pilot signals affected by fading and the known pilot signals P1(t), P2(t), . . . PN(t), whereby it is possible to estimate channel-impulse response vectors h1, h2, . . . hN of each of the pilot signals.
The control-amount calculation unit 13 uses these channel-impulse response vectors to calculate an amplitude and phase control vector (weight vector) w=[w1,w2, . . . wN]T of the transmit antennas 10-1 to 10-4 of the base station, which vector maximizes power P indicated by Equation (6) below. The vector is quantized, multiplexed onto the uplink channel signal as feedback information and transmitted to the side of the base station by the multiplexer 18.P=wHHHHw  (6)H=[h1,h2, . . . hN)  (7)In the case of FIG. 15 also it will suffice to transmit the values of w2, w3, . . . wN in an instance where w1 is obtained as w1=1. In actuality, multiplier MP1 for multiplying the downlink transmit data signal by the weight vector w1 is omitted in FIG. 15.
FIG. 16 is a diagram illustrating an example of the structural detail of the mobile station. It is assumed in FIG. 16 that the base station has four transmit antennas. First, a downlink data signal from the base station is received by the receive antenna 12 and sent to a data channel despreader 20 and pilot channel despreader 22. The data channel is despread by the data channel despreader 20 and the pilot channel by the pilot channel despreader 22. The despread pilot signal, which is the result of processing by the pilot channel despreader 22, is input to channel estimation units 23-1 to 23-4.
The channel estimation units 23-1 to 23-4 compare receive pilot signals P1′ to P4′ and the known pilot signals P1 to P4 in order to obtain the channel estimation values from the transmit antennas 10-1 to 10-4 of the base station to the receive antenna 12. The channel estimation units 23-1 to 23-4 obtain channel impulse responses h1 to h4, which indicate the state of amplitude and phase modulation ascribable to propagation of the receive pilot signals and input these responses to the control-amount calculation unit 13. The latter has a number of weight vectors capable of being transmitted as feedback information and uses these vectors to calculate power P, finds the weight vector that will give the maximum power P and adopts this vector as feedback information.
The channel estimation units 23-1 to 23-4 input the impulse responses of respective ones of the transmit antennas to a channel estimation unit 24. The latter obtains an overall impulse response h and inputs the response to a receiver 21 so that the response will be used in demodulation of the data channel. Further, the control-amount calculation unit 13 inputs the obtained weight vector to the multiplexer 18 as feedback information, and the multiplexer 18 multiplexes this feedback information and the transmit data signal. A data modulator 25 performs orthogonal modulation based upon the multiplexed data, and a spread-spectrum modulator 26 applies spread-spectrum modulation to transmit the data signal, which contains the feedback information, from the transmit antenna 14 to the base station.
FIG. 16 illustrates a method of performing synchronous detection using the channel response vectors h1, h2, . . . , hN, which have been obtained from the pilot channel, in order to demodulate the downlink receive data. In this case the channel estimation value used in synchronous detection of the data symbol in receiver 21 is calculated as follows:h=Hw  (8)where h represents the channel impulse response vector of the data channel obtained by combination in the receive antenna of the mobile station. The length of the vector is L.
The optimum weight of closed-loop transmit diversity is calculated as the weight that maximizes the power P indicated by Equation (1). In order to find the weight accurately, however, it is necessary to perform a comparison using a value of power P that has been averaged over a certain interval of time. The averaging interval is decided by the receive power of the pilot symbol, fading speed and feedback frequency, etc. That is, if the receive power of the pilot is low, the averaging interval must be lengthened in order to raise the weight accuracy. If the fading speed is low, then the weight can be found accurately by lengthening the averaging interval. Conversely, if the fading speed is high, the averaging interval must be set short. In any case, the averaging interval represents a delay time for finding the optimum weight.
Accordingly, when a base station with which a mobile station is communicating is changed over by handover and closed-loop transmit diversity is started anew, {circle around (1)} a delay is produced and is equivalent to a measurement interval needed to calculate the weight of the base-station antenna at the destination of handover or {circle around (2)} a satisfactory measurement cannot be assured immediately after changeover. Further, there is a delay (feedback delay) that lasts until the antenna weight calculated by the mobile station is multiplexed into the uplink channel signal as feedback information, transmitted to the base station and reflected as the weight of the transmit antenna. This delay also represents a delay up to the time the base station at the handover destination starts closed-loop transmit diversity. Furthermore, another problem is that owing to feedback delay, feedback information that has been transmitted from the mobile station immediately before changeover of the base station is processed as the weight of the base station at the handover destination.
FIG. 17 illustrates an example of the configuration of a conventional system in a case where handover is performed. This shows an example of a case where handover is performed between two base stations 1 and 2. Components identical with those shown in FIG. 13 are designated by like reference characters. All antennas of the base stations 1, 2 and of a mobile station 4 are used for both sending and receiving. Further, the feedback information extraction unit 16 and amplitude and phase controller 17 of FIG. 13 are integrated, provided additionally with an antenna assigning function and illustrated as an antenna assigning and weight control unit 12. Further, the base stations 1 and 2 are identical in structure. Handover is carried out by sending and receiving messages in a higher-order layer between the base stations 1, 2, a base control unit 3, which serves as a host device, and the mobile station 4.
The base stations 1 and 2 are each provided with two transceive antennas 10-1, 10-2 and 20-1, 20-2, respectively. In this case, the base station 1 is also capable of controlling w2 while holding w1 fixed and the base station 2 is capable of controlling w4 while holding w3 fixed.
The mobile station 4 receives only the pilot signals P1, P2 of the base station 1 with which it is currently communicating and calculates the optimum weights w1, w2 of transmit diversity. After a changeover is made to base station 2 by handover, the mobile station 4 starts calculating the antenna weights w3, w4 using the pilot signals P3, P4 of base station 2 at the handover destination.
FIG. 18 illustrates receive control timing and the flow of feedback control in a case where the mobile station 4 is handed over from base station 1 to base station 2. Here the interval over which weight is measured is one slot and the weight is fed back every slot. Further, feedback delay is assumed to be approximately a half slot. If calculation of weight is started immediately after a changeover is made from base station 1 to base station 2, then, as illustrated in FIG. 18, {circle around (1)} the measurement interval immediately after handover must be made the usual one-half slot in order to start transmit diversity from the beginning of the second slot (i.e., the measurement interval is short). Furthermore, {circle around (2)} since the weight that was fed back immediately before handover is the weight of base station 1, it is not used in the first slot immediately after handover and {circle around (3)} transmit diversity actually starts from the second slot, meaning that control is delayed by one slot.
Feedback delay DL is decided not only by feedback-information transmission delay and processing delay but also by the number of quantization bits of one antenna weight and number of feedback bits assigned to one slot. Now, if antenna weight is quantized by two bits and a single feedback bit is assigned to one slot, then the time needed to feed back one antenna weight will be equivalent to two slots. Furthermore, in the event that the number of transmit antennas of the base station is large, a feedback delay that is proportional to the number of antennas will occur because the control weights of the antennas are fed back in regular order. Accordingly, in a case where the base station has been changed over by handover, a large delay will occur until optimum weights of all antennas are fed back. In other words, immediately after the base station is changed over at handover, a long period of time is required for closed-loop transmit diversity to function fully and characteristics are degraded as a result.
The effects of degraded characteristics appear conspicuously in a situation where high-speed cell selection is performed. High-speed cell selection is a transmission scheme for selecting the base station having the highest receive power level from among a plurality of active base stations in soft handover (base stations communicating simultaneously with a mobile station in soft handover), transmitting data solely from this base station and selectively changing over this base station at high speed to such an extent that it will be possible to follow up fading. As a result, downlink interference is reduced and a stable reception power level can be assured in regard to soft handover in which data is being transmitted from a plurality of base stations simultaneously. However, since base station changeover takes place frequently in this case, there is an increase in the effects of characteristic degradation that occurs by the time closed-loop transmit diversity functions fully immediately following changeover, and a problem which results is that high-speed cell changeover gain is not obtained.